A raster registration system for a television camera is provided in which an error waveform is utilized to develop a correction waveform for reducing the misregistration of the rasters.
Color television cameras generally include two or more image pickup devices (image forming tubes or charge coupled device (CCD) arrays) for developing separate video signals representative of component colors of a scene being viewed by the camera. Accurate registration or synchronization of the video signals produced by a color television camera is important so that the signals simultaneously convey information related to the same portion of an image.
Camera registration has traditionally been accomplished by adding linear combinations of predetermined waveforms to best approximate the registration error of the camera. The weighting coefficients for these waveforms are entered by a technician who dials in varying amounts of the different waveforms while the camera is aimed at a test chart. These waveforms are then used to modify the deflection signals applied to the imaging devices to bring the signals provided by the various devices into alignment.
This manual approach and many automatic approaches typically require the use of calibration charts to construct the data set used for on-air correction. Automatic registration systems have been developed which automatically converge on the optimal set of adjustments while the camera is aimed at the test chart. This typically involves developing an output video waveform from each image pickup device by capturing images of the test chart and comparing the phase or time displacement of the resultant video waveforms with those produced by the other image pickup devices.
In other words, many known automatic registration systems require the use of a special registration setup chart to enable a predetermined pattern of figures to be imaged onto the various pickup devices of the camera, and registration is performed as part of the normal camera setup procedure prior to going on-air. As the pattern is scanned, transitions in the video signals associated with the edges of the figures are compared, and correction signals are applied to the scan circuits of the camera so that corresponding transitions in the different video signals occur simultaneously. The correct operation of the many systems is dependent upon the existence of video signal transitions in predetermined regions of the raster, and, in some cases, upon transitions between full black and full white levels. Over a period of time, however, registration can change, because of changes in temperature or voltage, or because of drift in electrical circuits, and the camera must be taken off-air to readjust the registration.
If zoom, focus and iris adjustments are taken into account, as they must be for lens chromatic aberration correction, an extremely tedious and time-consuming setup procedure is necessary to build the registration data set for all the possible combinations of lens settings. Thus, for this application, a system which builds its error data set from typical scene material is desirable.
Another approach which uses on-air measurement divides the raster into many zones, and then stores, in memory, the errors for each zone as they are detected. The correction waveforms are updated as data becomes available. While this method solves the problem of setting up the camera, it requires a very large memory to store all of the errors in each of the zones for all of the various zoom, focus, and iris positions. An automatic registration correction system requiring a large memory for storing all of the errors in each of the zones is described in U.S. Pat. No. 4,500,916, entitled "Automatic On-Air Registration System and Method for Color TV Camera", which is hereby incorporated by reference for its teachings on automatic correction of registration errors.
Although there are automatic registration system that operate on-air to correct registration errors, those systems have limited capability or other disadvantages which limit their usefulness. Examples include U.S. Pat. No. 4,133,003, entitled "Raster Registration System for a Television Camera", and U.S. Pat. No. 4,316,257, entitled "Dynamic Modification of Spatial Distortion Correction Capabilities of Scintillation Camera", which are hereby incorporated by reference for their teachings on correction of camera registration errors. Thus, it is desirable to provide improved systems and methods that are capable of operating on-air with actual scenes to automatically detect and correct registration errors in signals produced by color television cameras.